1. Field of the Invention.
The present invention relates to electronic devices which are implantable within the human body, such as cardiac stimulating pulse generators. In particular, the present invention is related to an improved receiver antenna for use in a programmable pulse generator.
2. Description of the Prior Art.
One important advance in health care and medical technology over the past several decades has been the development of implantable electromedical pulse generators which provide electrical stimuli to the heart or to other organs. Implantable pulse generators now find common use as heart pacemakers which are typically implanted within the chest beneath the patient's skin and which provide electrical pulse stimuli to the heart via one or more flexible leads. An electrode at the distal tip of the lead normally contacts heart tissue to supply the electrical pulse stimulation from the pulse generator.
Programmable heart pacemakers have been developed which allow a physician to change operating parameters of the pacemaker after it has been implanted. For example, the physician may wish to alter the stimulation rate of the pacemaker from that which was set at the time the pacemaker was implanted; may wish to alter the refractory period duration; or may wish to vary the mode of operation of the pacemaker from an R wave inhibited demand mode to an asynchronous mode, or vice versa.
To accommodate the need for altering operating parameters of a pacemaker after it has been implanted, programmable pacemakers have been developed. The physician uses an external programmer unit which provides a magnetic or electromagnetic signal. The programmer is placed over the patient's skin where the pacemaker has been implanted, and reprogramming signals are transmitted to the pacemaker. In one common type of programmable pacemaker, a magnetically actuated reed switch is contained within the pacemaker for receiving the magnetic or electromagnetic signals from the programmer.
While a reed switch meets the physical size restrictions for use in a pacemaker, its inherent slow response limits the rate at which reprogramming signals can be transmitted from the programmer to the pacemaker. In a pacemaker in which several different operating parameters can be programmed, the slow rate of transmission of information from the programmer to the pacemaker necessitated by a reed switch is highly undesirable.
An alternative to the reed switch as a receiver in a programmable pacemaker is an antenna small enough to fit within the housing of the pacemaker. Small antennas can, however, exhibit limited distance response, high directivity, and high sensitivity to axial misalignment between the axis of the transmitter coil and the axis of the antenna. This can make proper positioning and orientation of the pacemaker during implantations as well as proper positioning of the programmer unit over the implanted pacemaker during use very critical.
There is a continued need for improved programmable pulse generators, including pacemakers, which are highly insusceptible to electromagnetic noise, will reliably receive signals from the programmer and will reprogram the pulse generator operating parameters even when the pulse generator is slightly misaligned or tilted with respect to the implanted pulse generator, and do not require extensive and difficult to tune, receiver circuitry.